Gas turbine engines used to propel today's subsonic commercial aircraft are typically surrounded by a nacelle structure having a drooped or angled inlet opening aligned relative to the direction of the surrounding airflow. This is particularly true for those engines mounted under the wings of an aircraft, wherein the influence of the surrounding aircraft structure, airfoil angle of attack, and other aerodynamic factors result in a nacelle inlet droop angle of approximately 3-5 degrees with respect to the engine centerline.
Prior art nacelle structures include a double wall intake portion extending upstream of the gas turbine engine air inlet and having internal and external surfaces defined by the end points of radii drawn perpendicular to a linear centerline parallel to the encountered airflow. The nacelle opening divides the free airstream into an internal portion which is directed into the gas turbine engine air inlet and an external portion which flows around the nacelle structure. The external surface configuration is defined so as to reduce the occurrence of shock waves or separation, thereby avoiding the creation of undesirable drag forces.
The interior surface, also symmetrical about the linear drooped centerline, forms a duct for directing internal flow in a direction generally parallel to the drooped nacelle centerline, but abruptly turning at the engine inlet to the direction of the engine centerline. Experience has shown that this turn introduces a circumferential static pressure gradient in the engine inlet region which decreases engine efficiency as well as increases generated noise.
U.S. Pat. No. 4,220,171 issued Sept. 2, 1980, to Ruehr et al. recognized this deficiency in the prior art nacelle structure and discloses the improvement wherein the internal and external nacelle surfaces are defined by the end points of radii drawn perpendicular to a curved centerline. The curved centerline is parallel to the free airstream flow at the nacelle inlet opening and parallel to the engine centerline at the engine air inlet opening. The structure of Ruehr et al. thus smoothly turns the internal airflow prior to entering the engine inlet, increasing engine efficiency by reducing internal static pressure loss and noise generation.
The curved external nacelle surface of Ruehr et al., however, has been found not to produce the same benefit. By forming the exterior surface about a curved centerline, the Ruehr et al. nacelle increases the local air velocity along the upper forward portion of the nacelle, increasing the likelihood of forming local shock waves and/or boundary layer separation. As noted above, such phenomena increase the undesirable surface drag on the nacelle structure resulting in higher aircraft operating costs. What is needed is a nacelle structure which provides reduced external aerodynamic drag while simultaneously avoiding the creation of internal static pressure losses. Disclosure of the Invention
It is an object of the present invention to provide a nacelle structure internally configured to efficiently conduct a flow of air into a gas turbine engine mounted on an aircraft.
It is further an object of the present invention to provide a nacelle structure externally configured to reduce external aerodynamic drag and/or shock wave formation.
It is still further an object of the present invention to receive a flow of air at an angle with respect to the centerline of the gas turbine engine and to redirect the received air into the engine parallel to the engine centerline.
According to the present invention, an annular nacelle structure for a gas turbine engine disposed within a free flow of air extends upstream of the engine and has an inlet opening oriented perpendicularly with respect to the free airflow direction. Air received within the nacelle inlet opening is conducted through the nacelle and into the gas turbine engine by a duct formed by the nacelle internal surface. The internal surface is defined by the end points of radii extending perpendicularly outward from a curved centerline, the curved centerline further being colinear with the flow direction of the received air at the nacelle inlet and colinear with the gas turbine engine centerline at the engine air inlet.
The invention further provides an external nacelle surface defined by the end points of radii extending perpendicularly from a linear centerline, the linear centerline further being colinear with the flow direction of the received air at the nacelle inlet.
By providing separately developed internal and external aerodynamic surfaces for interacting with the respective internal and external airflow, the nacelle structure according to the present invention achieves efficient engine operation without the creation of external airflow shock waves. Each surface is thus shaped to perform its particular task without compromise, resulting in an overall fuel savings due to increased engine efficiency over prior art canted nacelle structures.
Both these and other objects will be apparent to those skilled in the art upon review of the following description and appended claims and drawing figures.